Thousands of holes are drilled in military and commercial aircraft to facilitate fastener installation. Fastener installation tolerances in military and commercial aircraft are very tight.
Out of tolerance holes can occur during manufacture of an aircraft. For instance, tolerances might be exceeded due to angularity of holes extending through a part (more prevalent in thicker parts) and mis-shaped holes.
During service of an aircraft, holes can become damaged and out of tolerance. For instance, hole shapes can change during service. These damaged and out of tolerance holes must be repaired to precise requirements in order for the aircraft to remain in service.
Damaged and out of tolerance holes may be repaired using a cutter (e.g., a drill bit, a reamer) and drill block. The drill block is positioned on a part, and aligns and guides the cutter to drill out the hole. If the hole is still out of tolerance after drilling (that is, the cutter doesn't clean-up all of the damaged area), the process is repeated with a larger cutter. The drilling is repeated until the hole is within tolerance, or until the hole is bushed, or until the part is scrapped.
This repair process is very tedious, especially for hard-to-drill materials. As many as two to three hours can be taken to repair a single hole, and hundreds or thousands of holes might have to be repaired on a single aircraft.
It is highly desirable to process a repair hole only once, and not have to drill more than once. Much time would be saved. Moreover, re-drilling would be avoided. If a part has to be re-drilled, that part might have a reduced life expectancy or it might have to be scrapped.